1. Technical Field
The disclosure relates to several fullerene-containing hemicarceplexes and a method of purifying fullerenes by using the same.
2. Description of Related Art
Because of their versatile configurations and attractive properties, fullerenes, including cylindrical carbon nanotubes (CNTs) and spherical and spheroidal buckyballs, have found applications in a wide range of fields, including materials science, chemistry, super- and semi-conducting physics, and biology. Ignoring CNTs, which lack uniform diameters or lengths, the most abundant structurally distinct species in a typical fullerene extract are two buckyballs, i.e. C60 and C70. Even though they have been investigated widely since their discovery in the 1980s, the practical applications of buckyballs have been limited by their poor solubilities in organic solvents; this characteristic has also seriously complicated their isolation and purification.
Several elegant methods have been developed for the isolation of the more-abundant C60 from fullerene extracts; in contrast, isolating the lower-in-symmetry and photovoltaically-more-interesting C70 in high purity from the same mixtures has been less straightforward. Indeed, tedious purification involving crystallization and/or high-performance liquid chromatography (HPLC) is frequently required to obtain high-purity C70, making it much less affordable than C60 of the same quality; accordingly, relatively limited research has been undertaken to discover and expand the practical applications of C70.
One attractive approach for the selective isolation of C70 involves exploiting its host-guest complexation behavior. Although a few judiciously designed synthetic host molecules do form complexes with C60 and C70 in solution, using such host-guest complexes as a means of separating mixtures of buckyballs (i.e., with high degrees of selectivity and stability) remains a challenge.
Unlike carcerands, which cannot release their entrapped guests, hemicarcerands allow sequestration of complementary guests (forming room temperature-isolatable hemicarceplexes) as well as their release at elevated temperatures. Although Cram first proposed, in 1995, that the internal space of a cavitand dimer might be a suitable host for C60 (Hemicarcerands with interiors potentially capable of binding large guests. J. Chem. Soc., Chem. Commun. 1085-1087 (1995)), hemicarcerands that can selectively imprison guests as big as C60 and C70 have never been realized previously, presumably because of difficulties in balancing the steric sizes and free energies of complexation of the host and guest components to allow selective sequestration and release of the guests.